Aims: AP30663 is a novel compound under development for pharmacological conversion of atrial fibrillation by targeting the small conductance Ca²⁺ activated K⁺ (K_Ca2) channel. The aim of this extension phase 1 study was to test AP30663 at higher single doses compared to the first-in-human trial. Methods: Sixteen healthy male volunteers were randomized into 2 cohorts: 6- and 8-mg/kg intravenous single-dose administration of AP30663 vs. placebo. Safety, pharmacokinetic and pharmacodynamic data were collected. Results: AP30663 was associated with mild and transient infusion site reactions with no clustering of other adverse events but with an estimated maximum mean QTcF interval prolongation of 45.2 ms (95% confidence interval 31.5–58.9) in the 6 mg/kg dose level and 50.4 ms (95% confidence interval 36.7–64.0) with 8 mg/kg. Pharmacokinetics was dose proportional with terminal half-life of around 3 h. Conclusion: AP30663 in doses up to 8 mg/kg was associated with mild and transient infusion site reactions and an increase of the QTcF interval. Supporting Information support that the QTc effect may be explained by an off-target inhibition of the I_Kr channel.